Post-growth spectral tuning of InGaAs/GaAs quantum dot infrared photodetectors

Abstract

Infrared photodetectors are essential in many industries and modern applications require devices with enhanced capabilities. High-performance detectors can be used for spectroscopy in medicine and environmental monitoring. Imaging scenarios include the identification of military targets and predicting equipment failure. These thermal imaging systems benefit from multicolour photodetectors. For example, some heat-seeking missiles incorporate two-colour HgCdTe arrays to discern target aircraft from decoy flares. Hyperspectral imaging describes the fusion of imaging and spectroscopy. These systems exhibit high spatial and spectral resolution, generally by dispersing different wavelengths onto a focal-plane array. Agricultural surveys, extraterrestrial exploration and medical procedures can all benefit from this powerful technique. High-end detectors in the mid-wavelength and long-wavelength infrared are usually made from HgCdTe alloys. These narrow-bandgap semiconductors exhibit favourable optoelectronic properties, however fabrication challenges lead to extravagant costs. In comparison, mature fabrication processes are available for III-V materials. Interband photodetectors made from these compounds are only sensitive at shorter infrared wavelengths. In recent years, intersubband devices have been developed for longer wavelengths and quantum well infrared photodetectors are now commercially available. Focal-plane arrays made from these structures are cheaper and the yield is better than with the HgCdTe technology. Quantum dot infrared photodetectors can also be fabricated from III-V materials. These architectures are inherently sensitive to normal-incidence radiation and have long carrier lifetimes, so they are expected to out-perform their quantum well counterparts. The devices studied here may be applicable to meteorology, atmospheric monitoring, molecular biology and medicine. High-quality quantum dots are normally grown by self-assembly and this restricts their size and composition. Hence, directly fabricating devices to operate at different wavelengths is an ongoing challenge. Post-growth techniques can instead be used to tailor the spectral response and two such approaches are considered in this thesis. Firstly, guided-mode resonances have been exploited in narrowband transmission filters. This design is agnostic to the detector technology and suitable for rugged environments. Germanium and calcium fluoride were selected for the dielectric layers and deposited films were thoroughly characterised. Guided-mode resonance filters based on photonic crystal slabs were integrated with quantum dot infrared photodetectors. The photoresponse of these devices was linearly tunable with the radius of the photonic crystal holes. These detectors are shown to be suitable for hyperspectral imaging with further optimisation of the device architectures. Intermixing shifts the response of InGaAs/GaAs quantum dot infrared photodetectors, so it is an effective approach to spectral tuning. Dielectric capping layers can be used to control the amount of intermixing and this allows multicolour detectors to be monolithically fabricated. In these studies, the compositional and thermomechanical properties of different dielectrics were measured. Preliminary intermixing experiments were performed on different heterostructures to extract the dominant physical processes. Ultimately, multicolour quantum dot infrared photodetectors were fabricated on a single sample. Silica was used to enhance intermixing through impurity-free vacancy disordering, whereas titania suppressed intermixing. Finally, the performance of each device was correlated with the properties of each dielectric. These detectors are found to be ideal for multispectral applications in the long-wavelength infrared band. Show more